1. Field of the Invention
This invention relates to the reworking of electrical circuit packages, and more particularly to methods and apparatus for the automated removal of lead wires from bonding pads to which they are connected so that a defective circuit package may be replaced with a new one.
2. Description of the Related Art
TAB (tape automated bonding) integrated circuit (IC) chips are generally formed with straight lead wires that extend laterally out from the chip. To mount and electrically connect the chip to a circuit board or other substrate, the chip is generally first secured to the substrate with an adhesive. The lead wires are then bent down to contact corresponding bonding pads that surround the chip, and are joined to the pads by a technique such as thermosonic, ultrasonic or thermo-compression bonding. If the circuit device should thereafter become damaged or otherwise malfunction, it is desirable that both it and its leads be removed from the substrate so that it can be replaced with a functional chip that uses the same contact pads.
If the leads were originally solder bonded, they need merely be heated to reflow the solder and remove the lead from the bonding pad. However, solder bonds are generally not as reliable as thermosonic, ultrasonic or thermo-compression bonds. For the latter type of bonding, two techniques have commonly been used to remove the leads of a defective chip from their respective bonding pads. In the first, illustrated in FIG. 1, the bonding pad 2 is elongated and the lead wire 4 is bonded to the pad at a location 6 towards the opposite end of the pad from the IC chip. To remove the lead wire, it is simply cleaved along a bend line 8 where it to joins the pad. The portion of the lead on the chip side of this line is then removed along with the chip, while the foot 10 of the lead is left bonded to the pad 2. The lead wire 12 for the replacement chip is bonded at a different location on the pad 2, inward from the residual foot 10 of the first lead. If the second chip also needs to be removed, its leads are severed in a manner similar to the leads of the first chip, leaving the foot of each replacement lead still attached to its respective bonding pad. A third lead wire 14 can then be bonded to the pad at a location along the pad that is still further inward towards the chip. While this rework method is subject to automation and is therefore relatively efficient in terms of the amount of time required, the elongated contact pads increase the amount of substrate area that must be dedicated to each separate chip. This results in a corresponding reduction in the circuit density that can be accommodated.
A second technique, illustrated in FIG. 2, allows for the use of a shorter contact pad 16 that can accommodate only a single lead. In this approach the lead for a defective chip is severed as in the technique of FIG. 1, leaving the foot 18 of the first lead attached to the bonding pad. The foot 20 of the next lead is then bonded directly over the foot 18 of the first lead. In FIG. 2 the assembly is illustrated after a second reworking, in which the foot 20 of the second lead has been severed and a third lead 22 has been bonded to the upper surface of second lead foot 20. While this approach allows for a higher circuit density than in FIG. 1, the compound lead bonding in which bonds are made to previous bonds results in a poor bonding reliability for the replacement leads.